Here's some information about gluten intolerance and celiac disease, which is an intolerance, not an allergy.
Gluten is found in a protein in certain grains, such as wheat, barley, and rye, as well as other products, like Brewer's yeast, etc. List of things that contain gluten. It may be found in small amounts in various other foods because of machines that process gluten-bearing foods along with others, such as oats. This is why you'll sometimes see "gluten free" on products that naturally have none - the machines are not used for gluten-bearing foods, and that's important to know if your intolerance is severe.
Unfortunately, not all medical practitioners and medical institutions, let alone insurance companies, recognize gluten intolerance.
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